1. Field of the Invention
The present invention generally relates to the manufacturing of self-aligned structures in multiple-layer devices. It more specifically relates to the manufacturing of a microtip cathode of a flat display screen.
The operating principle and the detail of the forming of an example of a microtip screen are described in U.S. Pat. No. 4,940,216 to the Commissariat à l'Energie Atomique to which reference will be made for any general teaching on this type of screen. Usually, a flat microtip screen is formed from two glass plates. The lower plate includes a microtip cathode structure, and one or several grid structures. The upper plate, arranged in operation to face the lower plate, supports an anode structure. The elementary microtips are arranged in various ways, and can be selectively addressed by acting upon perpendicular cathode and extraction grid lines. Generally, a large number of microtips are simultaneously addressed for each pixel of a screen.
The present invention more specifically aims at the forming of a screen of the type illustrated in FIG. 1. This screen includes a lower surface or cathode plate 1 and an upper plate or anode plate 2. The upper surface includes a layer, lines or pixels of phosphor material 3.
2. Discussion of the Related Art
On cathode plate 1, an upper layer corresponds to conductive cathode lines, possibly covered with a resistive material. On these cathode lines are formed microtips 5 in openings of an extraction grid 6. Extraction grid 6 is formed on a first insulating layer 7 formed on the upper surface of cathode 1. This upper surface will be said to correspond to the upper surface of the system substrate. Above grid layer 6 is formed a second insulating layer 8 in which a second conductive layer 9 corresponding to a focusing grid is laid. In this focusing grid and in second insulating layer 8 are formed openings which must be arranged precisely with respect to the openings formed in the extraction grid.
Various methods, for example described in French patent application 2,779,271 of the Commissariat à l'Energie Atomique, are known to form in a self-aligned manner the openings in the two metal levels 6 and 9 and in insulating layers 7 and 8. However, these methods appear in practice to be either inaccurate or difficult to implement. Further, these methods do not always enable independently and accurately adjusting the recess of the etching of the first insulating layer with respect to the second conductive layer and the recess of the etching of the second conductive layer with respect to the first conductive layer.